A duct shut-off device which is self-acting in the event of fire and which is intended particularly for use in ventilation ducts is known from SE-B-7602567-5 (Publication No. 401 614). This known shut-off or closure device is comprised of a grating made of supportive cellular material having cells which extend through the grating from one surface to another and which are defined by strip-like cell walls which are coated with a material which will swell when heated. When heated, this material will expand and block the cells, therewith effectively preventing fire from spreading through a ventilation duct in which the device is fitted for instance.
FR-A-2 254 182 teaches a similar device where the grating is comprised of a plurality of parallel ribs which extend over the cross-sectional area of the duct. One drawback with this device is that when the device is closed the resultant increase in pressure of the flowing medium will be concentrated over a short section of the duct with the subsequent danger of the duct rupturing or fracturing.